In my opinion, nothing. There are very few metal bands I would describe as mainstream or underground. A majority of them are somewhere in between the two in terms of how known they are, so I just don't label as anything.

I don't label bands as underground ever. A "underground" band doesn't go around selling out the House Of Blue. They may not be well known to the crowd of mayhem going there for FFDP and ITM but to say underground makes you sound stupid.